THE TITANIUM NITROGEN SYSTEM - A STUDY OF PHASE REACTIONS IN THE SUBNITRIDE REGION BY MEANS OF DIFFUSION COUPLES

An investigation of the Ti-N system by means of in situ diffusion couples was carried out in order to determine the phase equilibria in the subnitride region. Some of the thermal treatments were effected in a temperature gradient parallel to the diffusion bands, hence the metallgraphic microsections reflect the stability conditions in the corresponding temperature interval. The high-temperature phases eta-Ti3N2-x and zeta-Ti4N3-x could be observed as parallel diffusion layers. eta-Ti3N2-x is stable in the range 1066-1103-degrees-C and zeta-Ti4N3-x is stable in the range 1078-1291-degrees-C. The following five non-variant three-phase conodes were found by temperature gradient diffusion couples: alpha-Ti(N) + epsilon-Ti2N --> eta-Ti3N2-x (1066-degrees-C), eta-Ti3N2-x + epsilon-Ti2N --> zeta-Ti4N3-x (1078-degrees-C), epsilon-Ti2N --> zeta-Ti4N3-x + delta-TiN1-x (1080-degrees-C), alpha-Ti(N) + zeta-Ti4N3-x --> eta-Ti3N2-x (1103-degrees-C) and zeta-Ti4N3-x --> alpha-Ti(N) + delta-TiN1-x (1291-degrees-C). Nitrogen concentration profiles in the diffusion couples were determined by EPMA. Part of a tentative Ti-N phase diagram in the region 23-40 at.% N and 1020-1300-degrees-C is presented.